sys.dm_os_volume_stats

Returns information about the operating system volume

SELECT * FROM sys.dm_os_volumne_stats (database_id, file_id)

-- get data for all of the DBs:

SELECT ... 

FROM sys.master_files f 

CROSS APPLY sys.dm_os_volume_stats(f.database_id, f.file_id) vs

http://msdn.microsoft.com/en-us/library/hh223223.aspx

Note: Available from MSSQL 2008 R2 SP1.